This invention relates generally to integrated optical circuitry and, more particularly, to optical waveguide polarizers. In recent years, devices have been developed to make use of an electrooptical effect in which an electric field induces a change in refractive index of an electrooptic crystal. An optical field propagating in a waveguide formed in the crystal is phase-modulated by the induced change in refractive index. The electooptical effect can be used, for example, in modulators and in switches.
When electrooptical devices are used in conjunction with optical fibers, as is typically the case, a problem arises concerning the polarization state of the propagating waves. It is well known that, when linearly polarized light is coupled into a single-mode fiber, the light becomes slightly randomized in polarization, and there is conversion from linear to elliptical polarization. When light emerges from the fiber, it is highly probably that both orthogonal polarizations, known as the transverse magnetic (TM) and transverse electric (TE) modes in planar waveguides, will be excited and will be coupled to an associated electrooptical device. Such devices process different polarizations with different efficiencies, and performance degradation results when both orthogonal polarizations are present.
Basically, there are three possible solutions to the problem. One is to use specially constructed polarization-preserving fibers, such as fibers of elliptical cross-section, but this is clearly an expensive solution. A second solution is construct electrootical devices that are polarization-independent. The third solution, and the one with which the present invention is concerned, is to interpose some type of polarizer between the fiber and the electrooptical device.
Prior to this invention, waveguide polarizers have been of two basic types. One type of polarizer employs a material such as a metal cladding to provide a differential attenuation with respect to the TE and TM modes of polarization. One mode is absorbed more than the other, to achieve the desired differential attenuation, but in practice even the desired mode is attenuated by the cladding. The other approach is to use an anisotropic overlay or "superstrate" over the waveguide. The overlay is selected to provide a more effective cladding material for one mode than for the other. Accordingly the propagated energy in one mode tends to be transmitted through the polarizer, while the energy in the other mode tends to leak out of the waveguide. Again, there is a likehood that even the desired mode will be attenuated by the polarizer.
It will be appreciated from the foregoing that there is still a significant need for a waveguide polarizer for use in conjunction with electrooptical devices. The present invention is directed to this end.